Sputtering target and method for manufacturing semiconductor device

ABSTRACT

An object is to provide a deposition technique for depositing an oxide semiconductor film. Another object is to provide a method for manufacturing a highly reliable semiconductor element using the oxide semiconductor film. A novel sputtering target obtained by removing an alkali metal, an alkaline earth metal, and hydrogen that are impurities in a sputtering target used for deposition is used, whereby an oxide semiconductor film containing a small amount of those impurities can be deposited.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/474,365, filed Sep. 2, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 13/221,252, filed Aug. 30, 2011, now U.S. Pat.No. 8,835,214, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2010-197509 on Sep. 3, 2010,all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a sputtering target and a method formanufacturing the sputtering target. Further, the present inventionrelates to a method for manufacturing a semiconductor device which ismanufactured with the use of the sputtering target and uses an oxidesemiconductor.

In this specification, a semiconductor device means a general devicewhich can function by utilizing semiconductor characteristics, and anelectro-optic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

BACKGROUND ART

A transistor formed over a flat plate such as a glass substrate, whichis typically used in a liquid crystal display device, is generallyformed using a semiconductor material such as amorphous silicon orpolycrystalline silicon. A transistor manufactured using amorphoussilicon has low field effect mobility, but can be formed over a largerglass substrate. In contrast, a transistor manufactured usingpolycrystalline silicon has high field effect mobility, but needs acrystallization step such as laser annealing and is not always suitablefor a larger glass substrate.

Thus, a technique in which a transistor is manufactured using an oxidesemiconductor as a semiconductor material and applied to an electronicdevice or an optical device has attracted attention. For example, PatentDocument 1 and Patent Document 2 disclose a technique by which atransistor is formed using zinc oxide or an In—Ga—Zn-based oxidesemiconductor as a semiconductor material and such a transistor is usedas a switching element or the like of an image display device.

A transistor in which a channel formation region (also referred to as achannel region) is provided in an oxide semiconductor can have higherfield effect mobility than a transistor using amorphous silicon. Anoxide semiconductor film can be formed by a sputtering method or thelike at a relatively low temperature. Its manufacturing process iseasier than that of a transistor using polycrystalline silicon.

Transistors which are formed using such an oxide semiconductor over aglass substrate, a plastic substrate, or the like are expected to beapplied to display devices such as a liquid crystal display, anelectroluminescent display (also referred to as an EL display), andelectronic paper.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

However, characteristics of a semiconductor element which ismanufactured using an oxide semiconductor are not yet sufficient. Forexample, controlled threshold voltage, high operation speed, andsufficient reliability are required for a transistor including an oxidesemiconductor film.

An object of one embodiment of the present invention is to provide adeposition technique for forming an oxide semiconductor film. Inaddition, an object of one embodiment of the present invention is toprovide a method for manufacturing a highly reliable semiconductorelement using the oxide semiconductor film.

The density of carriers in an oxide semiconductor film has influence onthe threshold voltage of a transistor including the oxide semiconductor.The carriers in the oxide semiconductor film are generated due toimpurities contained in the oxide semiconductor film. For example,impurities such as a compound containing a hydrogen atom typified byH₂O, a compound containing an alkali metal, or a compound containing analkaline earth metal contained in the deposited oxide semiconductor filmincrease the carrier density of the oxide semiconductor film.

In order to achieve the above objects, impurities contained in the oxidesemiconductor film which have influence on the carrier density, forexample, a compound containing a hydrogen atom typified by H₂O, acompound containing an alkali metal, or a compound containing analkaline earth metal may be removed. Specifically, a novel sputteringtarget obtained by removing an alkali metal, an alkaline earth metal,and hydrogen that are impurities in a sputtering target used fordeposition is used, whereby an oxide semiconductor film containing asmall amount of those impurities can be deposited.

A sputtering target of one embodiment of the present invention is asputtering target for forming an oxide semiconductor film and includes asintered body of at least one oxide selected from zinc oxide, aluminumoxide, gallium oxide, indium oxide, and tin oxide. When observed bySIMS, the concentration of each of alkali metals contained in thesintered body is 5×10¹⁶ cm⁻³ or lower. Further, when observed by SIMS,the concentration of hydrogen contained in the sintered body is 1×10¹⁹cm⁻³ or lower, preferably 1×10¹⁸ cm⁻³ or lower, more preferably lowerthan 1×10¹⁶ cm⁻³.

More specifically, the concentration of sodium (Na) observed by SIMS is5×10¹⁶ cm⁻³ or lower, preferably 1×10¹⁶ cm⁻³ or lower, more preferably1×10¹⁵ cm⁻³ or lower. The concentration of lithium (Li) observed by SIMSis 5×10¹⁵ cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ or lower. Theconcentration of potassium (K) observed by SIMS is 5×10¹⁵ cm⁻³ or lower,preferably 1×10¹⁵ cm⁻³ or lower.

It has been pointed out that an oxide semiconductor is insensitive toimpurities, there is no problem when a considerable amount of metalimpurities is contained in the film, and therefore, soda-lime glasswhich contains a large amount of an alkali metal such as sodium and isinexpensive can also be used (Kamiya, Nomura, and Hosono, “CarrierTransport Properties and Electronic Structures of Amorphous OxideSemiconductors: The present status”, KOTAI BUTSURI (SOLID STATEPHYSICS), 2009, Vol. 44, pp. 621-633). But such consideration is notappropriate.

An alkali metal and an alkaline earth metal are adverse impurities for atransistor using an oxide semiconductor layer and are preferablycontained as little as possible. An alkali metal, in particular, Nadiffuses into an oxide and becomes Na⁺ when an insulating film incontact with the oxide semiconductor layer is an oxide. In addition, Nacuts a bond between metal and oxygen or enters the bond in the oxidesemiconductor layer. As a result, transistor characteristics deteriorate(e.g., the transistor becomes normally-on (the shift of thresholdvoltage to a negative side) or the mobility is decreased). In addition,this also causes variation in the characteristics. Such a problem issignificant especially in the case where the hydrogen concentration inthe oxide semiconductor layer is sufficiently low. Thus, theconcentration of an alkali metal is required to be set to the abovevalue in the case where the concentration of hydrogen in the oxidesemiconductor layer is 5×10¹⁹ cm⁻³ or lower, particularly 5×10¹⁸ cm⁻³ orlower.

Note that in this specification, a measurement value obtained bysecondary ion mass spectrometry (SIMS) is used as the impurityconcentration in the sputtering target or the oxide semiconductor film.It is known that it is difficult to obtain accurate data in theproximity of a surface of a sample or in the proximity of an interfacebetween stacked films formed using different materials by the SIMSanalysis in principle. Thus, in the case where distributions of theimpurity concentrations in the films in thickness directions areanalyzed by SIMS, the smallest value in a region where the films areprovided, the value is not greatly changed, and almost constant level ofstrength can be obtained is employed as the impurity concentration.Further, in the case where the thickness of the film is small, a regionwhere almost constant level of strength can be obtained cannot be foundin some cases due to the influence of the impurity concentration in thefilms adjacent to each other. In this case, the smallest value of theimpurity concentration in a region where the films are provided isemployed as the impurity concentration in the film.

In one embodiment of the present invention, a sputtering targetcontaining a small amount of impurities such as a hydrogen atom, analkali metal, and an alkaline earth metal can be provided. An oxidesemiconductor film in which impurities are reduced can be formed usingthe sputtering target. A method for manufacturing a highly reliablesemiconductor element including the oxide semiconductor film containinga small amount of impurities can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a flow chart illustrating a method for manufacturing asputtering target;

FIGS. 2A and 2B are top views illustrating a sputtering target; and

FIGS. 3A to 3E are cross-sectional views illustrating an example of amethod for manufacturing a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in detail belowwith reference to drawings. Note that the present invention is notlimited to the following description and it will be easily understood bythose skilled in the art that modes and details can be modified invarious ways. In addition, the present invention is not construed asbeing limited to description of the embodiment.

Embodiment 1

In this embodiment, a method for manufacturing a sputtering target whichis one embodiment of the present invention will be described withreference to FIG. 1. FIG. 1 is a flow chart illustrating an example of amethod for manufacturing a sputtering target according to thisembodiment.

First, plural kinds of single metals (Zn, In, Al, Sn, and the like) thatare materials of the sputtering target are each purified by repeatingdistillation, sublimation, or recrystallization (S101). After that,purified metals are each processed into a powder form. Note that in thecase of using Ga or Si as the material of the sputtering target, asingle crystal is obtained by a zone melt method or a Czochralski methodand then processing into a powder form is performed. Then, each of thesputtering target materials is baked in a high-purity oxygen atmosphereso as to be oxidized (S102). Subsequently, each of the oxide powders isweighed as appropriate, and the weighed oxide powders are mixed (S103).

The purity of the high-purity oxygen atmosphere is, for example, 6N(99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., theconcentration of impurities is 1 ppm or lower, preferably 0.1 ppm orlower).

In this embodiment, a sputtering target for an In—Ga—Zn-based oxidesemiconductor is to be manufactured. For example, In₂O₃, Ga₂O₃, and ZnOare weighed so that the composition ratio of In₂O₃:Ga₂O₃:ZnO is 1:1:1[molar ratio].

As examples of the sputtering target for an oxide semiconductor which ismanufactured in this embodiment, without being limited to a sputteringtarget for an In—Ga—Zn-based oxide semiconductor, the following can begiven: a sputtering target for an In—Sn—Ga—Zn-based oxide semiconductor,a sputtering target for an In—Sn—Zn-based oxide semiconductor, asputtering target for an In—Al—Zn-based oxide semiconductor, asputtering target for a Sn—Ga—Zn-based oxide semiconductor, a sputteringtarget for an Al—Ga—Zn-based oxide semiconductor, a sputtering targetfor a Sn—Al—Zn-based oxide semiconductor, a sputtering target for anIn—Zn-based oxide semiconductor, a sputtering target for a Sn—Zn-basedoxide semiconductor, a sputtering target for an Al—Zn-based oxidesemiconductor, a sputtering target for an In-based oxide semiconductor,a sputtering target for a Sn-based oxide semiconductor, a sputteringtarget for a Zn-based oxide semiconductor, and the like.

Next, the mixture is shaped into a predetermined shape and baked,whereby a sintered body of the metal oxide is obtained (S104). By bakingthe sputtering target material, hydrogen, moisture, hydrocarbon, and thelike can be prevented from being mixed into the sputtering target. Thebaking can be performed in an inert gas atmosphere (a nitrogenatmosphere or a rare gas atmosphere), in vacuum, or in a high-pressureatmosphere, and further, may be performed with application of mechanicalpressure. As a baking method, an atmospheric sintering method, apressure sintering method, or the like can be used as appropriate. Asthe pressure sintering method, a hot pressing method, a hot isostaticpressing (HIP) method, a discharge plasma sintering method, or an impactmethod is preferably used. Although the maximum temperature at whichbaking is performed is selected depending on the sintering temperatureof the sputtering target material, it is preferably set to approximately1000° C. to 2000° C., or more preferably, 1200° C. to 1500° C. Althoughthe holding time of the maximum temperature is selected depending on thesputtering target material, 0.5 hours to 3 hours is preferable.

Note that the filling rate of the sputtering target for an oxidesemiconductor in this embodiment is preferably higher than or equal to90% and lower than or equal to 100%, or more preferably, higher than orequal to 95% and lower than or equal to 99.9% inclusive.

Next, mechanical processing is performed in order to form a sputteringtarget having desired dimensions, a desired shape, and desired surfaceroughness (S105). As a processing means, for example, mechanicalpolishing, chemical mechanical polishing (CMP), or a combination ofthese can be used.

Then, in order to remove minute dust generated by the mechanicalprocessing and components of a grinding solution, the sputtering targetmay be cleaned. Note that, in the case where the sputtering target iscleaned by ultrasonic cleaning in which the target is soaked in water oran organic solvent, cleaning with running water, or the like, it ispreferable that heat treatment be subsequently performed forsufficiently reducing the concentration of hydrogen in the target or ona surface thereof.

Then, heat treatment is performed on the sputtering target (S106). Theheat treatment is preferably performed in an inert gas atmosphere (anitrogen atmosphere or a rare gas atmosphere). Although the temperatureof the heat treatment differs depending on the sputtering targetmaterial, it is set to a temperature at which the sputtering target isnot changed in property. Specifically, the temperature is higher than orequal to 150° C. and lower than or equal to 750° C., preferably, higherthan or equal to 425° C. and lower than or equal to 750° C. Heating timeis, specifically, 0.5 hours or longer, preferably, for an hour orlonger. The heat treatment may be performed in vacuum or in ahigh-pressure atmosphere.

After that, the sputtering target is attached to a metal plate called abacking plate (S107). A backing plate has functions of cooling thesputtering target material and being a sputtering electrode and thus ispreferably formed using copper, which is excellent in thermalconductivity and electric conductivity. Alternatively, titanium, acopper alloy, a stainless steel alloy, or the like can be used otherthan copper.

Further, at the time of attaching the sputtering target to the backingplate, the sputtering target may be divided and attached to one backingplate. FIGS. 2A and 2B illustrate examples in which the sputteringtarget is divided and attached (bonded) to one backing plate.

FIG. 2A illustrates an example in which a sputtering target 851 isdivided into four pieces of sputtering targets 851 a, 851 b, 851 c, and851 d and they are attached to a backing plate 850. FIG. 2B illustratesan example in which a sputtering target is divided to a larger number ofsputtering targets; that is, a sputtering target 852 is divided intonine pieces of sputtering targets 852 a, 852 b, 852 c, 852 d, 852 e, 852f, 852 g, 852 h, and 852 i, and they are attached to the backing plate850. Note that the number of pieces of divided sputtering targets andthe shape of the target are not limited to the number and the shape inthe case of FIG. 2A or FIG. 2B. When the sputtering target is divided,warpage of the sputtering target can be relaxed in the attachment of thesputtering target to the backing plate. In particular, when a film isformed over a large substrate, such divided sputtering targets can besuitably used for a sputtering target which is upsized in accordancewith the size of the large substrate. Needless to say, one sputteringtarget may be attached to one backing plate.

It is preferable that the sputtering target which has been subjected tothe heat treatment be transferred, stored, and the like in a high-purityoxygen gas atmosphere, a high-purity N₂O gas atmosphere, or an ultra dryair (having a dew point of −40° C. or lower, preferably −60° C. orlower) atmosphere, in order to prevent entry of impurities such asmoisture, hydrogen, or an alkali metal. The target may be covered with aprotective material formed of a material with low water permeabilitysuch as a stainless steel alloy, and the above gas may be introducedinto a gap between the protective material and the target. It ispreferable that the oxygen gas and the N₂O gas do not contain water,hydrogen, and the like. Alternatively, the purity of an oxygen gas or aN₂O gas is preferably 6N (99.9999%) or higher, more preferably 7N(99.99999%) or higher (that is, the impurity concentration in the oxygengas or the N₂O gas is 1 ppm or lower, preferably 0.1 ppm or lower).

Through the above process, the sputtering target described in thisembodiment can be manufactured. The sputtering target in this embodimentcan contain a small amount of impurities by using the materials each ofwhich is purified to have high purity in the manufacturing process.Further, the concentration of impurities contained in the oxidesemiconductor film which is formed using the target can also be reduced.

The above-described manufacture of the sputtering target is preferablyconducted in an inert gas atmosphere (a nitrogen atmosphere or a raregas atmosphere) without exposure to air.

Similarly, the sputtering target is set in a sputtering apparatus in aninert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere)without exposure to air. Accordingly, hydrogen, moisture, an alkalimetal, or the like can be prevented from attaching to the sputteringtarget.

In addition, after the sputtering target is set in the sputteringapparatus, dehydrogenation treatment is preferably performed to removehydrogen which remains on a surface of or inside the target material. Asthe dehydrogenation treatment, a method in which the inside of the filmformation chamber is heated to 200° C. to 600° C. under reducedpressure, a method in which introduction and removal of nitrogen or aninert gas are repeated while the inside of the film formation chamber isheated, and the like can be given.

Moreover, in the sputtering apparatus in which the sputtering target isset, it is preferable that the leakage rate be set to 1×10⁻¹° Pa·m³/secor lower, entry of water as an impurity be reduced with the use of,specifically, a cryopump as an evacuation unit, and counter flow be alsoprevented.

With reference to FIGS. 3A to 3E, an example will be described below inwhich a transistor is manufactured with the use of a sputteringapparatus in which a sputtering target manufactured in theabove-described process is set. Also in a manufacturing process of thetransistor, it is preferable that impurities such as a compoundcontaining a hydrogen atom typified by H₂O, a compound containing analkali metal, and a compound containing an alkaline earth metal beprevented from entering an oxide semiconductor film formed in thesputtering apparatus.

First, a conductive film is formed over a substrate 100 having aninsulating surface, and then, a gate electrode layer 112 is formedthrough a first photolithography process and an etching step.

An insulating film serving as a base film may be formed between thesubstrate 100 and the gate electrode layer 112; in this embodiment, abase film 101 is provided. The base film 101 has a function ofpreventing diffusion of impurity elements (e.g., Na) from the substrate100 and can be formed from a film selected from a silicon oxide film, asilicon oxynitride film, a silicon nitride film, a hafnium oxide film,an aluminum oxide film, a gallium oxide film, and a gallium aluminumoxide (Ga_(x)Al_(2-x)O_(3+y) (where x is greater than or equal to 0 andless than or equal to 2, and y is greater than 0 and less than 1)) film.By provision of the base film 101, impurity elements (e.g., Na) from thesubstrate 100 can be prevented from diffusing into an oxidesemiconductor film that is formed later. The structure of the base filmis not limited to a single-layer structure, and may be a layeredstructure of a plurality of the above films.

Then, a gate insulating layer 102 is formed over the gate electrodelayer 112 by a sputtering method or a PCVD method (see FIG. 3A). Also atthe time of formation of the gate insulating layer 102, it is preferablethat entry of impurities such as a compound containing an alkali metaland a compound containing an alkaline earth metal be prevented, and thegate insulating layer 102 is formed without exposure to air after thebase film 101 is formed.

Then, after the gate insulating layer 102 is formed, a first oxidesemiconductor film having a thickness of greater than or equal to 1 nmand less than or equal to 10 nm is formed over the gate insulating layer102 by a sputtering method without exposure to air. In this embodiment,the first oxide semiconductor film is formed to a thickness of 5 nm inan oxygen atmosphere, an argon atmosphere, or a mixed atmosphere ofargon and oxygen under conditions where a target for an oxidesemiconductor (a target for an In—Ga—Zn-based oxide semiconductorcontaining In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]) is used, thedistance between the substrate and the target is 170 mm, the substratetemperature is 250° C., the pressure is 0.4 Pa, and the direct current(DC) power source is 0.5 kW. The target for an oxide semiconductorincludes a sintered body of at least one oxide selected from zinc oxide,aluminum oxide, gallium oxide, indium oxide, and tin oxide, and theconcentration of each of alkali metals contained in the sintered body bySIMS is 5×10¹⁶ cm⁻³ or lower. In addition, the concentration of Nacontained in the sintered body by SIMS is 5×10¹⁶ cm⁻³ or lower,preferably 1×10¹⁶ cm⁻³ or lower, further preferably 1×10¹⁵ cm⁻³ orlower. In addition, the concentration of Li contained in the sinteredbody by SIMS is 5×10¹⁵ cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ or lower.In addition, the concentration of K contained in the sintered body bySIMS is 5×10¹⁵ cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ or lower.

After the first oxide semiconductor film is formed, without exposure toair, first heat treatment is performed by setting an atmosphere wherethe substrate is placed to a nitrogen atmosphere or dry air. Thetemperature of the first heat treatment is higher than or equal to 400°C. and lower than or equal to 750° C. In addition, heating time of thefirst heat treatment is longer than or equal to one minute and shorterthan or equal to 24 hours. By the first heat treatment, a firstcrystalline oxide semiconductor film 108 a is formed (see FIG. 3B).

Subsequently, after the first heat treatment, without exposure to air, asecond oxide semiconductor film having a thickness greater than 10 nm isformed over the first crystalline oxide semiconductor film 108 a by asputtering method. In this embodiment, the second oxide semiconductorfilm is formed to a thickness of 25 nm in an oxygen atmosphere, an argonatmosphere, or a mixed atmosphere of argon and oxygen under conditionswhere a target for an oxide semiconductor (a target for anIn—Ga—Zn-based oxide semiconductor containing In₂O₃, Ga₂O₃, and ZnO at1:1:2 [molar ratio]) is used, the distance between the substrate and thetarget is 170 mm, the substrate temperature is 400° C., the pressure is0.4 Pa, and the direct current (DC) power source is 0.5 kW. The targetfor an oxide semiconductor includes a sintered body of at least oneoxide selected from zinc oxide, aluminum oxide, gallium oxide, indiumoxide, and tin oxide, and the concentration of each of alkali metalscontained in the sintered body by SIMS is 5×10¹⁶ cm⁻³ or lower. Inaddition, the concentration of Na contained in the sintered body by SIMSis 5×10¹⁶ cm⁻³ or lower, preferably 1×10¹⁶ cm⁻³ or lower, furtherpreferably 1×10¹⁵ cm⁻³ or lower. In addition, the concentration of Licontained in the sintered body by SIMS is 5×10¹⁵ cm⁻³ or lower,preferably 1×10¹⁵ cm⁻³ or lower. In addition, the concentration of Kcontained in the sintered body by SIMS is 5×10¹⁵ cm⁻³ or lower,preferably 1×10¹⁵ cm⁻³ or lower.

Note that it is preferable that entry of impurities such as a compoundcontaining a hydrogen atom typified by H₂O, a compound containing analkali metal, and a compound containing an alkaline earth metal beprevented at the time of deposition of the first oxide semiconductorfilm and the second oxide semiconductor film. Specifically, the distance(also called a TS distance) between the substrate and the target is madelarge, in which case a high-mass impurity element is eliminated andentry thereof at the time of deposition is reduced; alternatively, thefilm formation chamber is set to a high vacuum state, in which case H₂Oor the like which is attached to the substrate is evaporated from asurface on which the film is formed. Further, it is preferable thatimpurities such as a compound containing a hydrogen atom typified byH₂O, a compound containing an alkali metal, and a compound containing analkaline earth metal be prevented from entering the oxide semiconductorfilm by setting the substrate temperature at the time of deposition tohigher than or equal to 250° C. and lower than or equal to 450° C.

After the second oxide semiconductor film is formed, without exposure toair, second heat treatment is performed by setting an atmosphere wherethe substrate is placed to a nitrogen atmosphere or dry air. Thetemperature of the second heat treatment is higher than or equal to 400°C. and lower than or equal to 750° C. In addition, heating time of thesecond heat treatment is longer than or equal to one minute and shorterthan or equal to 24 hours. By the second heat treatment, a secondcrystalline oxide semiconductor film 108 b is formed (see FIG. 3C).

Then, an oxide semiconductor stack of the first crystalline oxidesemiconductor film 108 a and the second crystalline oxide semiconductorfilm 108 b is processed into an island-shaped oxide semiconductor stack(see FIG. 3D).

The oxide semiconductor stack can be processed by etching after a maskhaving a desired shape is formed over the oxide semiconductor stack. Theabove mask can be formed by a method such as photolithography.Alternatively, a method such as an inkjet method may be used to form themask.

For the etching of the oxide semiconductor stack, either wet etching ordry etching may be employed. It is needless to say that these may becombined.

After that, a conductive film for forming a source electrode layer and adrain electrode layer (including a wiring formed in the same layer asthe source electrode layer and the drain electrode layer) is formed overthe oxide semiconductor stack and is processed to form a sourceelectrode layer 104 a and a drain electrode layer 104 b.

Next, an insulating film 110 a and an insulating film 110 b which coverthe oxide semiconductor stack, the source electrode layer 104 a, and thedrain electrode layer 104 b are formed (see FIG. 3E). The insulatingfilm 110 a is formed using an oxide insulating material, and afterdeposition, third heat treatment is preferably performed. By the thirdheat treatment, oxygen is supplied from the insulating film 110 a to theoxide semiconductor stack. The third heat treatment is performed in aninert atmosphere, an oxygen atmosphere, a mixed atmosphere of oxygen andnitrogen, at a temperature higher than or equal to 200° C. and lowerthan or equal to 400° C., preferably higher than or equal to 250° C. andlower than or equal to 320° C. In addition, heating time of the thirdheat treatment is longer than or equal to one minute and shorter than orequal to 24 hours.

Through the above process, a bottom-gate transistor 150 is formed.

The transistor 150 includes the base film 101, the gate electrode layer112, the gate insulating layer 102, the oxide semiconductor stackincluding a channel formation region, the source electrode layer 104 a,the drain electrode layer 104 b, and the insulating film 110 a, whichare formed over the substrate 100 having the insulating surface. Thesource electrode layer 104 a and the drain electrode layer 104 b areprovided to cover the oxide semiconductor stack. A region functioning asthe channel formation region is part of the oxide semiconductor stackwhich overlaps with the gate electrode layer 112 with the gateinsulating layer 102 interposed therebetween.

In a semiconductor layer (which refers to the above oxide semiconductorstack) including the channel formation region of the transistor 150which is illustrated in FIG. 3E, the concentration of Na observed bySIMS is 5×10¹⁶ cm⁻³ or lower, preferably 1×10¹⁶ cm⁻³ or lower, furtherpreferably 1×10¹⁵ cm⁻³ or lower. In the semiconductor layer includingthe channel formation region of the transistor 150, the concentration ofLi observed by SIMS is 5×10¹⁵ cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ orlower. In the semiconductor layer including the channel formation regionof the transistor 150, the concentration of K observed by SIMS is 5×10¹⁵cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ or lower.

In the semiconductor layer including the channel formation region of thetransistor 150 illustrated in FIG. 3E, it is preferable that theconcentration of hydrogen observed by SIMS be 5×10¹⁹ cm⁻³ or lower,specifically 5×10¹⁸ cm⁻³ or lower.

The semiconductor layer including the channel formation region of in thetransistor 150 illustrated in FIG. 3E is a stack of the firstcrystalline oxide semiconductor film 108 a and the second crystallineoxide semiconductor film 108 b. The first crystalline oxidesemiconductor film 108 a and the second crystalline oxide semiconductorfilm 108 b are c-axis aligned. Note that the first crystalline oxidesemiconductor film 108 a and the second crystalline oxide semiconductorfilm 108 b comprise an oxide including crystals with c-axis alignment(also referred to as C-Axis Aligned Crystal (CAAC)), which has neither asingle crystal structure nor an amorphous structure. The firstcrystalline oxide semiconductor film 108 a and the second crystallineoxide semiconductor film 108 b partly include a crystal grain boundaryand are apparently different from an amorphous oxide semiconductor film.

In the case of the transistor including a stack of a first crystallineoxide semiconductor film and a second crystalline oxide semiconductorfilm, the amount of change in threshold voltage of the transistorbetween before and after a bias-temperature (BT) stress test can bereduced even when the transistor is irradiated with light; thus, such atransistor has stable electric characteristics.

Note that although an example of a bottom-gate transistor is illustratedin FIGS. 3A to 3E, the present invention is not limited thereto and atop-gate transistor can also be manufactured, for example.

REFERENCE NUMERALS

-   100: substrate, 101: base film, 102: gate insulating layer, 104 a:    source electrode layer, 104 b: drain electrode layer, 108 a: first    crystalline oxide semiconductor film, 108 b: second crystalline    oxide semiconductor film, 110 a: insulating film, 110 b: insulating    film, 112: gate electrode layer, 150: transistor, 850: backing    plate, 851: sputtering target, 851 a: sputtering target, 851 b:    sputtering target, 851 c: sputtering target, 851 d: sputtering    target, 852: sputtering target, 852 a: sputtering target, 852 b:    sputtering target, 852 c: sputtering target, 852 d: sputtering    target, 852 e: sputtering target, 852 f: sputtering target, 852 g:    sputtering target, 852 h: sputtering target, and 852 i: sputtering    target

This application is based on Japanese Patent Application serial no.2010-197509 filed with Japan Patent Office on Sep. 3, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A sputtering target for forming an oxide semiconductorfilm, comprising: a sintered body of at least one oxide selected fromzinc oxide, aluminum oxide, gallium oxide, indium oxide, and tin oxide,wherein concentration of each of alkali metals contained in the sinteredbody by SIMS is 5×10¹⁶ cm⁻³ or lower.
 3. The sputtering target accordingto claim 2, wherein concentration of sodium contained in the sinteredbody by SIMS is 1×10¹⁶ cm⁻³ or lower.
 4. The sputtering target accordingto claim 2, wherein concentration of sodium contained in the sinteredbody by SIMS is 1×10¹⁵ cm⁻³ or lower.
 5. The sputtering target accordingto claim 2, wherein concentration of lithium contained in the sinteredbody by SIMS is 5×10¹⁵ cm⁻³ or lower.
 6. The sputtering target accordingto claim 2, wherein concentration of lithium contained in the sinteredbody by SIMS is 1×10¹⁵ cm⁻³ or lower.
 7. The sputtering target accordingto claim 2, wherein concentration of potassium contained in the sinteredbody by SIMS is 5×10¹⁵ cm⁻³ or lower.
 8. The sputtering target accordingto claim 2, wherein concentration of potassium contained in the sinteredbody by SIMS is 1×10¹⁵ cm⁻³ or lower.
 9. A sputtering target for formingan oxide semiconductor film, comprising: a sintered body of at least oneoxide selected from zinc oxide, aluminum oxide, gallium oxide, indiumoxide, and tin oxide, wherein concentration of each of alkali metalscontained in the sintered body by SIMS is 5×10¹⁶ cm⁻³ or lower, andwherein concentration of hydrogen contained in the sintered body by SIMSis 1×10¹⁹ cm⁻³ or lower.
 10. The sputtering target according to claim 9,wherein concentration of sodium contained in the sintered body by SIMSis 1×10¹⁶ cm⁻³ or lower.
 11. The sputtering target according to claim 9,wherein concentration of sodium contained in the sintered body by SIMSis 1×10¹⁵ cm⁻³ or lower.
 12. The sputtering target according to claim 9,wherein concentration of lithium contained in the sintered body by SIMSis 5×10¹⁵ cm⁻³ or lower.
 13. The sputtering target according to claim 9,wherein concentration of lithium contained in the sintered body by SIMSis 1×10¹⁵ cm⁻³ or lower.
 14. The sputtering target according to claim 9,wherein concentration of potassium contained in the sintered body bySIMS is 5×10¹⁵ cm⁻³ or lower.
 15. The sputtering target according toclaim 9, wherein concentration of potassium contained in the sinteredbody by SIMS is 1×10¹⁵ cm⁻³ or lower.